1. Field of the Invention
The present invention relates to a clutch device, a motor apparatus and a wiper system.
2. Description of Related Art
For example, a wiper system, which wipes a window glass of a vehicle, has a motor apparatus as its drive source.
The wiper motor apparatus, particularly a rear wiper motor apparatus, includes a worm wheel, a swing arm and a joint member. The worm wheel is meshed with a rotatable shaft of an armature, which is rotatably supported by a case. The swing arm is connected to a wiper shaft. One end of the joint member is connected to a predetermined point of the worm wheel, and the other end of the joint member is connected to the swing arm. Upon energization of the wiper motor apparatus, the armature is rotated to rotate the worm wheel. Then, the rotation of the worm wheel is converted to a swing motion of the swing arm through the joint member to reciprocally rotate the wiper shaft. Therefore, a wiper arm, which is directly installed to the wiper shaft, is swung to wipe a rear glass of the vehicle. One such wiper motor apparatus is disclosed in Japanese Unexamined Patent Publication No. H09-118202.
In the non-operating state of the wiper where a wiper blade connected to the wiper arm is stopped and is thus held substantially parallel to a lower edge of the glass surface, when an external force, which is exerted by, for example, heavy snow, is applied to the wiper blade and the wiper arm, the wiper blade and the wiper arm are pressed downward beyond a lower turning point. At this time, the wiper shaft and the swing arm of the wiper motor apparatus are rotated further beyond a normal reciprocal rotational angular range. Therefore, the joint member, which is connected to the swing arm, or the worm wheel, which is connected to the joint member, could be damaged by the external force.
Thus, to limit the damage of the above components and to limit rotation of the wiper arm into a vehicle body region beyond the window glass area of the vehicle, in the wiper motor apparatus disclosed in Japanese Unexamined Patent Publication No. H09-118202, a rotational range limiting portion is provided at a location, which is outside of the normal reciprocal rotational angular range of the swing arm to limit the rotation of the swing arm beyond the predetermined reciprocal rotational angular range of the swing arm.
However, an excessively large external force could be applied to, for example, the wiper blade and the wiper arm even in a case where the wiper blade and the wiper arm are not placed outside of the predetermined reciprocal rotational angular range. In other words, the excessively large external force could be applied to, for example, the wiper blade and the wiper arm even in the case where the wiper blade and the wiper arm are operated within the predetermined reciprocal rotational angular range (a normal wiping range). This could be a case where heavy snow, which has been accumulated on a roof of the vehicle, falls onto the wiper blade and the wiper arm, which is operated and is located in the normal wiping range other than the lower turning point. In such a case, the wiper blade and the wiper arm are arrested by the fallen heavy snow or receive an excessively large external force from the fallen snow. Therefore, the excessively large external force is applied to the swing arm, the joint member, the worm wheel and/or the worm gear through the wiper shaft, so that these components could be damaged by the excessively large external force. Thus, the above-described disadvantage still exists.
Therefore, in the case of the wiper motor apparatus described in Japanese Unexamined Patent Publication No. H09-118202, each corresponding component of the wiper motor apparatus needs to be designed to withstand the above described excessively large external force.
It has been proposed to provide a frictional clutch mechanism to an output shaft (the wiper shaft) to limit damage to the corresponding component of the wiper motor apparatus, such as the worm wheel, or to limit burnout of the wiper motor apparatus even at the time of application of the excessively large external force. However, such a frictional clutch mechanism significantly disadvantageously reduces a transmission efficiency of the rotational force. Furthermore, a noise could be disadvantageously generated through frictional slide movement of the clutch. Also, an activation point of the frictional clutch mechanism is largely depend on the frictional force of the frictional clutch mechanism, so that the activation point of the frictional clutch mechanism may vary from product to product.
To address the above disadvantage, it is conceivable to provide a clutch, which is operated through meshed engagement, to the output shaft to protect the entire structure of the wiper motor apparatus including the swing mechanism, the speed reducing mechanism and the motor main body at the time of application of the excessively large external force during the operation of the wiper blade and the wiper arm in the normal rotational angular range (in the normal wiping operation). In this type of clutch, which is operated through the meshed engagement, it is possible to limit damage to the corresponding component of the wiper motor apparatus and to limit burnout of the wiper motor apparatus at the time of locking the output shaft. Also, the loss of the rotational force by the sliding movement is small, so that the transmission efficiency of the rotational force is increased. In addition, the generation of the noise is limited.
However, in the case of the clutch, which is operated through the meshed engagement, when an excessively large external force (an excessively large load) is applied to the clutch, the meshed engagement is released. When the meshed engagement of the clutch is released, relative rotation occurs between the output shaft (the driven side component) and the worm wheel (the driving side component or the motor side component). Thus, for example, in a case where the wiping position of the wiper arm, which is secured to the output shaft, is controlled by an automatic stop device, which is provided to the worm wheel (the driving side component or the motor side component) to automatically stop the wiper arm at a predetermined stop position, when the excessively large external force is applied to the clutch to cause the release of the meshed engagement, the predetermined stop position of the wiper arm is displaced. Thus, the wiper arm cannot be automatically stopped at the predetermined stop position.